The development of optical communications systems using silica-based optical fibers has stimulated interest in light sources and photodetectors capable of operating within the wavelength region, generally between approximately 0.7 .mu.m (micrometer) and approximately 1.6 .mu.m, of interest to such systems. A photodetector is an essential component of such a system and as a result, much effort has been directed toward developing structures and materials for photodetectors.
The photodetectors presently contemplated for use in fiber-based optical communications systems fall into three general categories. First, there are p-i-n photodiodes. However, these suffer the drawback of not having any current gain. Second, avalanche photodiodes have been developed and these, of course, have current gain. However, the avalanche process introduces noise which is undesirable for many communications systems purposes. Further, a large bias voltage, typically more than 30 volts, is generally required to obtain avalanching. Third, high sensitivity phototransistors have been developed. However, these devices generally have low optical gain in the low incident power region where optical communications systems operate and where high gain is most critically needed. Further, the response time for phototransistors is often slower than that desired because of minority carrier storage in the base region.
The problem of minority carrier storage and the resulting undesirably slow response time can be alleviated, if not totally eliminated, by using majority carrier devices which lack minority carrier storage and will give faster response times. Perhaps the first and best known majority carrier diode is the Schottky barrier diode which has a metal-semiconductor interface. Other types of majority carrier devices have been developed recently. For example, Applied Physics Letters, 35, pp. 63-65, July 1, 1979 reported a majority carrier device which, perhaps in a flight of fancy, the author termed a camel diode. The device was so designated because there is a hump in the conduction band formed by a thin and highly doped p-type layer that controls the carrier transport. The doping concentration in this layer is such that it is fully depleted of holes at all bias values. Another version of this device was reported in Electronics Letters, 16, pp. 836-838, Oct. 23, 1980. This device has the p-type layer which controls the carrier transport embedded between nominally undoped semiconductor layers.
The devices reported in both articles, however, are rectifying devices. Although the latter article suggested that the devices could be used as mixer diodes, no other uses are suggested in either article and the devices disclosed in both articles are not suitable for use as photodetectors.